Before we saw this, some of our readers wrote to us to say it was fantastic. Some of them wrote to say it was rubbish. And you know what? All of them were right.

For the first twentysomething minutes, it scarcely puts a foot wrong. The opening scene, with the still-visible Bad Wolf and Rose's telling response of "Bye" to Mickey's "I love you", sets the scene nicely, and the Doctor and Rose's joy in each other's company is beautifully judged. It's easy for that kind of stuff to be a bit squirmy (it often was with the Ninth Doctor), but here it seems totally natural.

And aside from the horrific matte paintings, the New Earth hospital's an intriguing setup. The Sisters of Plenitude look absolutely astonishing, the Face of Boe's a welcome return visitor, and the disinfection sequence is 24-carat comedy gold. In fact, there are hilarious lines aplenty in here – we love the Doctor’s “Think of what you look like to them - all pink and yellow” and Cassandra’s “What do you think I’m going to do? Flap you to death?”. David Tennant fully lives up to the potential he showed in The Christmas Invasion, and is without question the Doctor in a way Christopher Eccleston never managed. (And thank God for an actor who can do comedy. It makes all the difference.) We weren't all that jazzed to see Cassandra again - it seemed to us that whatever interest she had was fully used up in The End Of The World - but Zoe Wanamaker is so bloody good at it that we were prepared to make an exception.

Then Rose meets Cassandra, and things plummet downhill like a luge on a glacier.

It's not that it's uniformly terrible from there on in: in fact, there are a lot of good things. Billie Piper does a bang-up job as Cassandra: we didn't know she, er, had it in her. We said in our review of the previous series finale that we hoped Rose would have new things to do this series, and so far, Russell T Davies has answered our plea with a vengeance.

Aside from the sections where he appears to be channelling Kenneth Williams, the Doctor's also amazing, dazzling us with one emotion after another while staying believably Doctorish. His rage, his concern, his quips all hit exactly the right note, and he's as proactive as all get-out. Love it. (Special big ups to both Russell T Davies and to David Tennant for the way the Doctor handles The Kiss: it couldn't be more perfect. Compare that with the dreadful car-crash train-wreck kiss in the Eighth Doctor Movie: see, if it's done right, the Doctor can survive a kiss without bringing the franchise down around his ears.)

And there are some good ideas too: the question of whether force-grown humans are "real" humans is without doubt a Big Worthy Weighty Theme, as is the question of the ethics of experimentation for medical research. (Hey, Kazuo Ishiguro used similar material for Never Let Me Go, and he got a Booker nomination, so this must be quality stuff.) The idea of curing rather than killing zombies, too, is uniquely Doctorly.

But it's not what you've got, it's what you do with it.

Like anyone with a smidgen of taste and discernment, we’re Avengers fans. And the thing about The Avengers, see, is that the characters are utterly awesome (Steed, if you’re reading this, call us! Yes, we are free on Saturday night) and the direction is ultimately styly: forty years on, those things stand up as well as ever. What lets The Avengers down, though, is the plotting: the plots are as full of holes as your granny’s lace tablecloth. Most of the time, if you think about them for more than half a second, they just don’t make sense.

Is this sounding familiar at all?

That, alas, is the problem with New Earth. The Doctor and Rose are irreproachably brilliant, and the secondary characters are great too. The direction might not be as cool as in the Avengers, but it makes up for it with some slammin’ special effects and makeup. The plot, though. Uh.

Take all the clone/experimentation stuff. This is set up brilliantly: the chat the Doctor has with the Sister looking after the Face of Boe leaves us in no doubt of her genuine compassion, and it's clear the Sisters are saving lives like no one else has ever managed. That makes the coldness with which they treat their guinea pigs even more shocking, and when you figure out what's going on it's like a punch in the solar plexus.

So why does it all fall apart? First of all, it's one thing to raise a big complex question, and another to grapple with it. There aren't any obvious answers here: the Sisters are doing a great deal of good with the result of their experimentation, and the Sisters' conversation about "sentience transfer" makes it clear that the Sisters don't think their subjects are sentient. Modern-day humans tend to go along with the idea that using non-sentient creatures for medical experimentation is a lesser evil than failing to stop disease, and that's why animal experimentation is widespread. None of us actually like the idea, but on the other hand none of us like the idea of people we love dying from otherwise-curable diseases either. The situation on New Earth is pretty much the same: the subjects are a lot more human, but the cures are a lot more spectacular. Is it OK to use thousands of force-grown humans if it cures all disease? What if it was just dozens? Or just one? And if that's not OK, why is it so much more OK to do it to animals? And what about the billions who will die otherwise?

Hell, we don't know. Neither do you, probably. And that's the point: it's an interesting question because it's a tough question. So what does Russell T Davies do with it? He feeds us the Doctor's opinion as the only answer, and that's that. It's Just Wrong? Oh, OK, then. The end of the story, with the Sisters being led away by the fuzz, is clearly supposed to be another example of the Doctor putting everything right, but nobody stops to consider the people who are going to die as a result. Bottled it.

And how does Russell T Davies shows us this theme in action? He makes the guinea pigs into zombies. They're so blatantly George A Romero that it's an obvious homage: bleargh, we say. As far as we're concerned, "homage" is another word for "too lazy and/or creatively bankrupt to think up any new ideas". Why would we want to see something we've seen again and again to the point of cliche before? And it's not just Romero it's mining, either: we had the same zombie theme and the same idea of the disease and cure being spread by touch in The Empty Child. Boring, boring, boring.

If the unoriginality of the zombies weren't enough to cripple them as a plot idea, the rampant plot stupidity gives them a good kicking and leaves them whimpering in a corner. Why, exactly, does Cassandra let them out in the first place? For people with every disease in the world, aren't they awfully mobile? How come the other hospital patients need the cures administered by IV but the zombies are cured just by a touch? If as the Doctor says viruses are still continuing to evolve, there must be some new diseases the Sisters haven't found a cure for yet, which is why they continue to experiment - so how come the zombies are all cured? And if they’re desperate to be touched, why the hell don’t they touch each other?

Then there's the Cassandra plot: this is mishandled in so many ways it makes us want to cry. The whole point of Cassandra as a sheet of skin is that that was deliberate: she did it perfectly willingly so that she could be as thin as possible. Her desperation to get a body not only lacks logic, it completely crumbles her impact in The End Of The World.

And that's just the beginning. What's she doing skulking around in a hospital, anyway? Why is she so interested in what's going on at the hospital? Since it's obvious the Doctor isn't going to be thrilled at the idea of her stealing Rose's body, why does she hang around smooching him instead of getting as far away from him as possible? (You may think there’s an obvious answer to that, and if you do, we’re with you all the way, but still.) How does she manage to skip from body to body, and more to the point, how does she manage to take over the Doctor so easily? He’s never been that much of a pushover before. If she's happy switching to Igor's body, why didn't she do that in the first place?

But that's not the worst part. After the awful-in-their-own-right obligatory Cassandra Learns Compassion scenes, the worst part comes at the end, when microseconds after sobbing and saying she doesn't want to die, Cassandra suddenly decides that's just peachy after all. Huh? Was there a fault in the broadcast? Did they miss out a couple of episodes in which Cassandra learns to come to terms with the finiteness of life? To top it off, this is capped by a heartstring-yanking death scene which left us spluttering for air in a tidal wave of mush. None of this plot made the tiniest pixel of sense, either logically or, even worse, emotionally. And when that happens, it's pretty much impossible for an episode to come out with its dignity intact.

So the themes are unexplored and the plot crumbly, but what gets to us more than that, more than anything, is the lack of originality. It’s not even a new planet: it’s just Earth transplanted, and the hospital might just as well be Station One. It’s supposed to be the far future, and people are still wearing the same clothes they are now. We see the same characters as in End Of The World; even the cute little camera spider’s a direct lift. The “patients” are stacked in rows, just like in Ark In Space, Tomb of the Cybermen and even the most recent episode The Christmas Invasion. And what is different – the zombies and the bodyswap – are dusty SF cliches rubbed thin from overuse. If you can’t think of anything new, Russell T Davies, why don’t you step aside from the writing part and get in writers who can?

A great first half, a knockout Doctor and a great and revitalised companion: these are the things we're hanging onto. They've got to be a good omen.

The lonely god? Jaysus. That’s a bit of a worry. What with that and the stuff about him being the highest authority, we’re wondering if pride goeth before a fall.

BUT I’M A LAYDEE

Chip is a fantastic character – memorably creepy as he strokes Cassandra, then acts the Doctor off the set when channelling her.

DANGEROUS CURVES

Why is Cassandra moaning about being a chav when she knew she was jumping into Rose? And didn’t that bouncy castle scene go on a bit? Yet again, we weren’t the target audience, but that and the cleavage gave us Peri flashbacks.

BY THE SKIN OF HER TEETH

The next time we see Cassandra’s frame after she’s jumped into Rose, her trampoline has gone. Where is it?

PLAYING PATIENTS

That Marconi’s Disease patient? All red and floating? Fantastic. And so is the delightfully incongruous plastic telephone hood.

SCREW YOU

Why is the Doctor threatening Cassandra with the sonic screwdriver? It’s Rose’s body he’d be blowing a hole in!

WHATEVER YOU SAY

When the Doctor orders Cassandra to leave Rose on the ladder, she does it without arguing, even though she knows she’s only jumped into Rose so the Doctor can use the sonic screwdriver.

A LONG WAY DOWN

The derring-do with the winch is pure magic. We particularly like the screaming.

TODAY IS A GOOD DAY TO HAVE AN ICE CREAM AND GO TO THE MOVIES

The Face Of Boe says “There are better things to do today. Dying can wait.” Not Klingon, then, despite the bumpy forehead.